Re: [PATCH] Rename/export combine_insn_cost as insn_rtx_cost

[Zack Weinberg <zack at codesourcery dot com>]

Roger Sayle <roger@eyesopen.com> writes:

>> extern void foo(void);
>>
>> void bar(int n) { if(n) foo(); }
>
>
> The problem is two-fold.  The first is that the "then" block in this
> function gets extened from just the single "call" instruction at
> .26.postreload, to include the following additional instruction by
> .28.flow2.
>
> (insn 33 32 21 1 (set (reg:DI 1 r1)
>         (reg:DI 34 r35)) 5 {*movdi_internal} (nil)
>     (expr_list:REG_DEAD (reg:DI 34 r35)
>         (nil)))
>
> I'm guessing that this something to do with the IA-64's ABI, even
> though the function doesn't return a value?

Right, that's the ABI-required reload of the global pointer.

> In theory, this is still manageable, if-conversion can make both
> instructions conditional on the same condition, provided that the
> condition isn't modified by the first instruction.  And there's
> the rub...
>
> ifcvt.c's cond_exec_process_insns calls modified_in_p to check
> whether "(eq (reg:BI 262 p6 [340]) (const_int 0 [0x0]))" is
> modified
[...]
>               /* We'd like to test call_used_regs here, but rtlanal.c can't
>                  reference that variable due to its use in genattrtab.  So
>                  we'll just be more conservative.
[...]
> So rather than use call_used_regs to check whether "p6" is clobbered
> by the function call, or can be used as an EPIC qualifying predicate,
> we assume the worst.  I've no idea whether on IA-64 "p6" is call-saved
> or not, but the combination of above events block the transformation.
>
> Without knowing anything more about IA-64, isn't there some way of
> untangling rtlanal.c from the gen*.c programs, such that rtx_set_p
> can use call_used_regs.  Perhaps an IN_GENATTRTAB macro, or linking
> a dummy call_used_regs to genattrtab.c that's initialized to true
> for all hard registers??

My immediate reaction to the above comment was 'what on earth is
genattrtab doing using rtlanal.c?' and then I went and looked and
... it isn't!  rtlanal.o is not linked into genattrtab, or any other
gen* program, and egrep 'reg_set(_between)?_p' gen*.[ch] returns
nothing.  So I think we could just go ahead and use call_used_regs
there.

Unfortunately, p6 *is* call-clobbered on ia64.  HP's acc (which can do
this transformation) uses the call-saved p1 instead.  But if-conversion
happens after register allocation, so there would be no way to get the
register allocator to do that, unless I'm missing something?

zw